1. Field of the Invention
The present invention relates to a radar system mounted on a vehicle for measuring a range rate, e.g. a relative velocity and a relative distance between the vehicle and a preceding vehicle, obstacle etc.
2. Description of Related Art
A radio radar using millimeter waves and other waves has been used for air-traffic control, meteorological observation because of low propagation loss and great propagation distance even in bad weathers, e.g. rains, fogs. Thanks to these merits, in collision prevention and safety fields of automobiles, a millimeter wave radar to measure a vehicular gap (relative distance; car-to-car distance) and a relative vehicle velocity between a preceding vehicle and the following vehicle (the vehicle with the radar system) is being researched, developed and commercialized these days. A radar system of a diplex Doppler type is disclosed as a typical one of radar modulation systems in Japanese Application Patent Laid-open Publication No. Sho 49-107491.
Referring to FIG. 7, this radar system modulates a signal output from a millimeter wave oscillator 101 into two time-sharing signals on frequencies f1 and f2 (the difference xcex94f: f2xe2x80x94f1) by a modulation signal 102 from a modulator 103. These signals are transmitted by a transmit antenna 104 and reflect back when they meet a preceding vehicle 202, and then they are received by a receive antenna 106.
When there is some range rate (relative velocity) V between the preceding vehicle 202 and the millimeter wave radar system 100b of the following vehicle, Doppler frequencies fd1 and fd2 are caused in the reflected signals. As the result, the signals received by the receive antenna 106 have frequencies f1+fd2 and f2+fd2. A mixer 108 turns these signals into time-shared signals (or intermediate frequency signals 107 hereinafter abbreviated as IF signals) respectively containing information of Doppler frequencies fd1 and fd2. The IF signals 107 are amplified by an amplifier 109 and distributed to two low-pass filters (LPFs) 111 and 111 by a switch 110 which works in synchronism with the modulation signal 102.
Referring to FIG. 8, a relationship between transmit signals and IF signals 107 is explained below. Transmit signals consist of two time-sharing signals on frequencies f1 and f2. When the IF signals 107 pass through the mixer 108, frequency components f1 and f2 of the transmit signals are removed from the received signal and the IF signals 107 respectively become signals containing information of Doppler frequencies. As already described above, the switch 110 works in synchronism with the modulation signal 102 to distribute the IF signals 107 containing information of Doppler frequencies fd1 and fd2 into a signal of Doppler frequency fd1 and a signal of Doppler frequency fd2.
These Doppler signals are digitized by an A/D converter 112, and transformed (analyzed) with FFT (Fast Fourier Transform) by a DSP (digital signal processor) 113b. With the FFT analysis, Doppler frequencies fd1, fd2 and the phase differences xcfx861, xcfx862 are obtained. The relative velocity V between the preceding vehicle 202 and the following vehicle (the vehicle having this radar system) is expressed by equation (1) or (2) below.
V=(Cxc3x97fd1)/(2xc3x97f1)xe2x80x83xe2x80x83(1) 
or
V=(Cxc3x97fd2)/(2xc3x97f2)xe2x80x83xe2x80x83(2) 
wherein
C is a signal ray (propagation) velocity.
Let""s assume that fd1 less than  less than f1, fd2 less than  less than f2, and xcex94f less than  less than f1. In this case, fd1 can be approximately equal to fd2 and the relative velocity V can be expressed by equation (3) below.
V≈(Cxc3x97fd1)/(2xc3x97f0)xe2x80x83xe2x80x83(3) 
wherein
f0=(f1+f2)/2 
The relative distance R (range) between two cars can be expressed by equation (4) below.
R=Cxc3x97(xcfx861xe2x88x92xcfx862)/(4xcfx80xc3x97xcex94f)xe2x80x83xe2x80x83(4) 
After computing the relative velocity V and the relative distance (vehicular gap; car-to-car distance) R, the DSP (digital signal processor) 113b send them to ACC (adaptive cruise control) unit 120 of the vehicle through a system microcomputer 115b. 
For example, as shown in FIG. 2, let assume that a preceding vehicle 202 runs before a vehicle 201 with this radar system, these velocities are respectively V1 and V2, namely the relative velocity is (V1-V2), and the frequencies of Doppler signals are fd1 and fd2. FIG. 3 shows the result of the FFT analysis of the signals. In FIG. 3, a peak spectrum appears at the Doppler frequencies fd1 and fd2 on the Frequency axis. The relative velocity (V1-V2) and the relative distance R (DL in FIG. 2) between the preceding vehicle 202 and the following vehicle 201 can be obtained from this frequency information and the phase information as it is mentioned above.
This diplex Doppler type signal processing enables stable detection of a preceding vehicle without any complicated signal processing because it detects a spectrum corresponding to a preceding vehicle from the result of FFT analysis and we can get a relative velocity from the frequency information and a relative distance from the phase information simultaneously.
The above prior art has problems listed below.
For example, if the difference between the velocity V1 of a vehicle 201 and the velocity V2 of a preceding vehicle 202 as described referring to FIG. 2, that is, a relative velocity (V1-V2) is almost close to 0 and smaller than a minimum relative velocity that can be analyzed by the FFT, the frequency fd of the Doppler signal of the preceding vehicle 202 does not appear in the result of the FFT analysis as indicated in FIG. 9.
In other words, if the relative velocity of the preceding vehicle 202 to the following vehicle 201 is very small, there is a problem that this radar system cannot detect (catch) a preceding vehicle.
The present invention has been created considering the above problems, and the object of the present invention is to provide a radar system which can detect a preceding vehicle (object) even when the range rate of the preceding vehicle relative to the vehicle equipped with this system is very small.
To accomplish the aforesaid object, the present invention is characterized by
a radar system comprising a transmit antenna for alternately transmitting two signals on different frequencies, a receive antenna for receiving signals which were transmitted from said transmit antenna and reflected back on the preceding object (e.g. vehicle, obstacle) when they met an object, a first digitizing means for sampling and digitizing respectively said received signals, and a first analyzing means for analyzing the frequencies of said digitized receive signals, extracting a peak spectrum, and computing a range rate of said object from the frequency of said peak spectrum;
wherein said radar system further comprises a second digitizing means for sampling and digitizing respectively said received signals, a second analyzing means for analyzing the frequencies of said receive signals digitized by said second analyzing means, extracting a peak spectrum, and computing a range rate of said object from the frequency of said peak spectrum, and an output means for outputting selectively either the range rate computed by said first analyzing means or the range rate computed by said second analyzing means; said first analyzing means stores, as a threshold, a frequency which is within an analyzable low frequency range and higher than a minimum analyzable frequency; said second digitizing means samples and digitizes said received signals at a higher sampling resolution than the sampling resolution of said first digitizing means so that said second analyzing means may be able to analyze frequencies below said threshold; and said first analyzing means causes said second digitizing means and said second analyzing means to work when said received signals have a peak spectrum below said threshold.
In case that there is a single object before the vehicle having this radar system, when the second digitizing means and the second analyzing means work, the output means outputs the range rate of the preceding object (a relative velocity and a relative distance between 2 cars etc.) which is computed by the second analyzing means. When the second digitizing means and the second analyzing means do not work, the output means outputs the range rate of the object which is computed by the first analyzing means. In other words, when there is only one preceding object before the vehicle having this radar system, the output means selectively outputs either the range rate computed by the first analyzing means or the range rate computed by the second analyzing means. In case that there are two or more objects, for example, when the second digitizing means and the second analyzing means work for one of the objects, the output means outputs the range rate of the object which is computed by the second analyzing means. When the second digitizing means and the second analyzing means do not work for the other objects, the output means outputs the range rate of the object which is computed by the first analyzing means. In other words, when two or more objects are before the vehicle having this system, the output means can output both the range rate by the first analyzing means and the range rate by the second analyzing means.
The above radar system can be equipped with a variable-gain amplifying means for amplifying the received signals and a means for setting a gain of the amplifying means before the received signals is fed to the first digitizing means and/or the second digitizing means.
The above radar system can be also characterized in that said second analyzing means analyzes the frequencies of said received signals, extracts a peak spectrum, checks whether the frequency of the peak spectrum is below said threshold, and temporarily stores the effect when the frequency of the peak spectrum is below said threshold; and said first analyzing means, at next cycle after the frequency analysis of said received signals by said second analyzing means, analyzes the frequencies of said received signals, as a result even when there is no frequency of the peak spectrum below said threshold, said first analyzing means causes said second digitizing means and said second analyzing means to work if said second analyzing means stores the effect that the frequency of the peak spectrum is below said threshold.